Process for the production of zinc



June 21, 1949. s. ROBSON' 2,473,611

' PROCESS FOR THE PRODUCTION OF ZINC Filed Jun 5, 1947 Patented June 21,1949 UNITED STATES PATENT OFFICE PROCESS FGR THE PRODUCTION OF ZINCApplication June 5, 1947, Serial No. 752,668 In Great Britain June 5,1946 3 Claims.

This invention relates to the production of metallic zinc from roastedores, slags and/ or other materials containing zinc.

A process already in successful operation for the recovery of zinc fromslags containing zinc oxide comprises injecting pulverized coal with anair blast into a bath of molten slag; the primary reaction producescarbon monoxide with a little carbon dioxide; the CO reacts with thezinc oxide liberating zinc and forming more C02. The remaining C0 andzinc are then completely burnt by excess air, producing zinc oxide andfurther C02. The zinc oxide and gases thus produced are then cooled bypassing them through a steam generator, and the oxide is collected. Theprocess is essentially a batch as distinguished from a continuousprocess. Molten slag, together with the shell from the charging pots, ischarged into the furnace, and after a period of operation the spent slagis discharged. Existing plants for operating this process handle slagfrom lead blast furnaces containing about 16% zinc and 2% lead. The zincrecovery obtained is of the order of 95%. The charge is mainly moltenslag; this can be partly replaced by granulated slag, if necessary, butonly at the cost of increased fuel consumption.

The above mentioned process leads to the separation of zinc as zincoxide. My object is the recovery of metallic zinc instead of the oxideand the possibility of using the slagging furnace for this purpose hasbeen very thoroughly studied by me.

I have found that by no adaptation of the known process in which an airblast is used can a useful yield'of metallic zinc be obtained, since inorder to maintain the temperature of the slag bath an amount of air mustbe used substantially exceeding that required to burn the carbon tocarbon monoxide; much CO2 is therefore formed which, as the gases cool,and particularly in the process of condensation, re-oxidises most of thereleased metallic zinc vapour to zinc oxide, by reversal of thereduction reaction The need for excess air, if the bath temperature isto be maintained, arises from the dilution of the oxygen of the airblast by the inert nitrogen content of the air itself, which absorbs alarge proportion of the heat generated.

I have found that by using oxygen or an oxygen-rich gas mixture insteadof air, the total oxygen content of the blast relative to the totalcarbon content of the fuel supplied .can be reduced to such an extentthat CO2 is not formed in any appreciable quantity, without decreasingthe rate of evolution of heat to a point at which it ceases to balancethe heat losses and the slag ceases to be fluid; and that withprogressive enrichment of the blast gas in oxygen the total amount ofoxygen needed to keep the process going can be progressively decreasedbelow that required for complete combustion of the carbon to CO. Thezinc oxide in the slag is then reduced either directly by the excesscarbon in accordance with the reaction:

or partly by this reaction (2) and partly by the CO in accordance withreaction (1), the CO2 thereby produced being reduced by the excesscarbon thus:

The resultant effect of operating with an excess of carbon over theamount which can just be burnt to CO by the oxygen in the blast is toinhibit the production of CO2, and the greater the ex-' cess of carbonthe more successfully is the production of CO2 inhibited. By minimisingthe evolution of CO2, the extent to which the reverse of reaction (1)can occur during cooling is also minimised. It thus becomes possible toobtain a good yield of metallic zinc on condensation.

In the process according to my invention, a blast of gas containing notless than 40% by volume of oxygen, the remainder being substantiallyfree from C02 and composed principally of inert gas such as nitrogen,together with carbonaceous fuel in a finely divided state in suspensionin the blast gas, is blown into a bath of molten zinciferous slag whichmay include other materials containing zinc oxide or/and additionalcarbon, the molecular proportion of carbon to oxygen in the blast beinggreater than two, so that carbon is always present in suflicient excessin relation to the oxygen supply to preclude the formation of anysubstantial quantity of CO2.

By this process metallic zinc vapour is evolved mixed with a gascontaining a high proportion of carbon monoxide to carbon dioxide; thismixture can be collected with exclusion of air, and from it the zincvapour can be condensed as molten metal in a suitable condenser.

The efiiciencies both of liberation of zinc and of the condensation ofthe zinc vapour fall ofi rapidly with increasing dilution of the oxygencontent of the blast gas, the minimum oxygen concentration in the blastgas with which the process can be worked being about 40% as aboveevolved gases, with inert and heat absorbing 'lo gases or vapours andhenceitaisdesirablethat, in addition to the use of a high oxygenconcentration in the blast, the fuel shouldbe dry'and have a lowhydrogen contentybothhydrogen and water vapour constituteuselessdiiuents which absorb heat by reason of their thermal capacities,and also because, in the case of water vapounheat is absorbedendothermically in the reaction with carbon.

For this reason suitable fuels for the process 'of 'th'is inventionincludeanthracite and artificially carbonized fuels.

On'theotherhand when working-with a blast consisting-of substantiallypure oxygen (98% purity-or over) some dilutionof the products ofcombustion can be tolerated. It is therefore possible when -operatingwith a substantially pure oxygen blast to use liquid hydrocarbon 'fuelsbithe heavier types in which the proportion'of carbon to hydrogen ishigh. Such fuels may well-be useful because of '-theease or atomisingthem in the blast.

An instance of theapplication of the invention is to the production ofzinc metal from roasted zinc blende. a tank provided'with-tuyeresbelowthe level'to which it is to be filled with slag and preferably near thebottom. A tap-hole. or the'like atthe level at which the slag 'is'to berun off and a charging. device, by: which :solid materials: can= beintroduced without admitting. any alargeam'ount .Ofiairare-alsoprovided. .:'For the removal: of 'the zinc-vapour andtgases,-.theresis-1an::outlet in the upp rlpart of the tankaleading toa-condenser.

iTo' startlthe: operatiom the tank is :first 'partiallyfilled with: acharge of granulated: material of composition zsimilarxto zthat: of itheliquid: slag which will l be;maintained;in :the bath during thesubsequent: operations. 1 Pulverised' fuel, :with:ox-

ygen--rich-gas;:is .blowntin .through'tthe tuyeres. 2

When .a pool of vmolten-aslag has :been produced, steady operationcommences. 'iThe :charge' may consist of .a roastedpre, .=preerably?free from dust; cor. a ,zinciferous slag. erg: from a lead blastfurnaceyalternatively an-oreithat has -beenroast- .edand briquettedmayvbe used; the charge=may also consist oizaumixture ofa'oxidised oreand carbonaceousmfuel inthe form of briquettes. Fresh icharge material:which may be hot or cold, but.preferablylheated to 800? Csor: over, -isadded either continuouslyhn at short intervals. Slag- .producingmaterials such as :silica or/ and lime overzand above such 'as are'present in the-ore and -in the, :pulverised fuel .may-be:addedasrequired. Thefurnace :.becomessfilled i with*slag containing such-I1I'0p0rti0n:-'0fi zino'aszmay permit bf :high ratevofa'throughsputwith 1 an economic elimination. Zinc vapour is evolved= andis conducted to the-condenser. The=process-wdrks continuously.

T :Inlthe event.- ofnthe roasted orebeing briquetted with. carbonaceous:materiali the reaction between the-oxygen and carbon-in the blastserves'only tosgeneratezheat; supplying the requ'irements ofthenreactiombetwe'ent the zinc ox'ide and carbon The, furnace comprisesessentially in the briquettes in accordance with reaction (2). Thebriquettes should be formed so that they slag and disperse as the zincis eliminated.

In the process for recovering zinc as metal, as already described, thewhole slag bath will be maintained at constant composition, the zinccontent being high enough to allow: rapid removal-of the zinc but'low'en'ough' to 'avoid excessive loss by carrying over zinc oxide withthe spent slag.

Instead of .the furnace having a single chamber in which the whole ofthe slag is contained and can become thoroughly mixed, it may beconstructed'so that. the slag flows either through a "labyrinth orthrough a succession of chambers arranged so'that the zinc content fallsoff as the islag fiowsfrom the charging point to the tapping point..This may enable the rate of removal of the zinc from the slag-to beincreased and the zinc oxide content of the outflowing spent slag to belowered.

In certain circumstances the maximum oflimination of zinc asmetal'mayhot'be economical, inwhich case the extraction of zinc as metalmay be limited, "leaving some'zinc'oxide in the'spent slagdrawn on fromthe b'athpwhichmay 'then be treated for extraction of 'zincas oxide, e.g. by a knownprocessof thekind; previously mentioned.

If the charge materials- -contain any'appreciable amount of copper,silver or other'like metals. it-may be convenient .to adda sulphurcontaining material such asiron-sulphide to the charge which willgiverise to formation of a matte containing the" copper etc. which WilLsink'to the bottom of the bath and can be drawn off through a: second taphole' at the bottom of .the bath, which'must in any case be providedforfcleaning purposes and thev like. This second tap'hole will alsoenable any molten iron that may be formed to be drawn off.

It. is 'found that the gases as they. leave .the slag bath arerelatively. hot, and that. their sensible heat can be utilised foreffecting further reduction of zincfrom unslagged material. Thus it .issometimes convenient to pass the -zinc-contain ing gases as they leavethe bath and before they enter: the: condensenfthrough a heated columnof-briquettes containing roasted ore and carbon, whereby some'zinc isliberated. by .reaction'ilZ) at the expense of, a drop in temperature of.the gases. This column also has some beneficialeffeet in filtering outdroplets of slag entrained-in the gas.

Alternatively the gases; before .they..enter. i the condenser, may bepassedithrougha column of coke. 'This helpgto'ffiltermout droplets ofslag and also serves to reduce 'at 1east partially any CO2 that may be.formed byreaction .(3), notwithstanding :the excessof carbon.intheblast. To effect any considerable amount of carbon'dioxideremoval-it-will usually be necessary to'he'at the coke otherwise than.by ithe gases," for instance by meansof an electric current, using'thecoke as a resistor.

The accompanying drawingsillustrate a typical example of a suitable.furnaceior... carrying; out the process of jthe.inv.ention. Inthesedrawings:

i'Fig: 1' is.'a sectional-elevation of..the furnace along itslongitudinal. centraline;

Fig. 2 is a sectional elevation onthe line 2-2 ofFigJl;

'Fig. 3'. is a sectional (plan view l onJthe linels) 3--3 of Fig.landFigIZ.

' In"thefurnaceillustrated a tank for holding a pool 2 of molten slaghas a solid, refractory, slagresistant bottom 4, made e. g. of ceramicor carbonaceous material, such as sintered alumina, graphite orcarborundum, and hollow watercooled walls 5, the lower parts of whichaccommodate tuyeres 3. The tank includes a charging bay I, and thegas-collector space 8 in the furnace above the surface level of the slagbath 2 is sealed off from the opening of the charging bay by a part ofthe wall 5 which dips below the surface level of the bath. The walls ofthe gas collector space 8 are of solid refractory material and arecontinuous with an arched crown l0. An extension of the gas-collectorspace forms a flue 9 with a sloping floor ensuring that the slag carriedforward as spray and deposited in the line runs back into the tank; anda downcomer l 1 leads from the flue 9 into the condenser (not shown).The level of the slag bath is kept constant by a continuous run-offopening provided with a spout I, and a plugged tap-hole 6 at the bottomof the tank enables any molten iron or matte that is formed to bedrawn-off when required, and provides for complete emptying of the tank,when necessary.

I claim:

1. A process for the production of metallic zinc from roasted ores,slags and other zinciferous materials which comprises reducing thezinciferous material at an elevated temperature sufiicient to producemetallic zinc vapor in a nonoxidizing gaseom atmosphere substantiallyfree from carbon dioxide by blowing into a molten slag bath of thezinciferous material a blast of carbonaceous iuel suspended in anoxygen-containing gas comprising not less than 40% by volume of oxygenand the balance essentially inert gas substantially free from carbondioxide, the molecular proportion of carbon to oxygen in the blast beinggreater than two so that carbon is always present in sufficient excessin relation to the oxygen supply in said blast to preclude the existencein the zinc vapor-containing atmosphere of any substantial quantity ofcarbon dioxide, and condensing the resulting zinc vapor.

2. A process for the production of metallic zinc as set forth in claim 1which includes maintaining the bath of molten slag at substantiallyconstant composition by additions thereto of fresh charged materials,withdrawing spent slag from the bath at a level below the introductionof the blast gas in amount equivalent to the additions of fresh chargedmaterials, and substan tially continuously blowing a blast ofcarbonaceous fuel suspended in the oxygen-containing gas into the bathof molten slag.

3. A process for the production of metallic zinc as set forth in claim 1in which the oxygen-com taining gas comprises about 98% by volume of oxyen.

STANLEY ROBSON.

REFERENCES CITED The following referenlces are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,583,933 Kirby May 11, 19261,782,418 Garred Nov. 25, 1930

